This invention relates to a method for applying a thermal barrier coating to a metal substrate, or for repairing a previously applied thermal barrier coating on a metal substrate, of an article, in particular turbine engine components such as combustor deflector plates and assemblies, nozzles and the like. This invention further relates to a method for applying a thermal barrier coating, or repairing a previously applied thermal barrier coating, by plasma spray techniques where the underlying metal substrate has an overlaying aluminide diffusion coating.
Higher operating temperatures of gas turbine engines are continuously sought in order to increase their efficiency. However, as operating temperatures increase, the high temperature durability of the components of the engine must correspondingly increase. Significant advances in high temperature capabilities have been achieved through formulation of nickel and cobalt-base superalloys, though such alloys alone are often inadequate to form components located in certain sections of a gas turbine engine, such as turbine blades and vanes, turbine shrouds, buckets, nozzles, combustion liners and deflector plates, augmentors and the like. A common solution is to thermally insulate such components in order to minimize their service temperatures. For this purpose, thermal barrier coatings applied over the metal substrate of turbine components exposed to such high surface temperatures have found wide use.
To be effective, thermal barrier coatings should have low thermal conductivity (i.e., should thermally insulate the underlying metal substrate), strongly adhere to the metal substrate of the turbine component and remain adherent throughout many heating and cooling cycles. This latter requirement is particularly demanding due to the different coefficients of thermal expansion between materials having low thermal conductivity and superalloy materials typically used to form the metal substrate of the turbine component. Thermal barrier coatings capable of satisfying these requirements typically comprise a ceramic layer that overlays the metal substrate. Various ceramic materials have been employed as the ceramic layer, for example, chemically (metal oxide) stabilized zirconias such as yttria-stabilized zirconia, scandia-stabilized zirconia, calcia-stabilized zirconia, and magnesia-stabilized zirconia. The thermal barrier coating of choice is typically a yttria-stabilized zirconia ceramic coating, such as, for example, about 7% yttria and about 93% zirconia.
In order to promote adhesion of the ceramic layer to the underlying metal substrate and to prevent oxidation thereof, a bond coat layer is typically formed on the metal substrate from an oxidation-resistant overlay alloy coating such as MCrAlY where M can be iron, cobalt and/or nickel, or from an oxidation-resistant diffusion coating such as an aluminide, for example, nickel aluminide and platinum aluminide. To achieve greater temperature-thermal cycle time capability to increase servicing intervals, as well as the temperature capability of turbine components such as combustor splash or deflector plates of combustor (dome) assemblies, combustor nozzles and the like, an aluminide diffusion coating is initially applied to the metal substrate, typically by chemical vapor phase deposition (CVD). A ceramic layer is then typically applied to this aluminide coating by physical vapor deposition (PVD), such as electron beam physical vapor deposition (EB-PVD), to provide the thermal barrier coating. Usually, the various parts of the component (e.g., the deflector plates attached or joined to supporting structure such as the swirlers and backplate to form the combustor dome assembly, or airfoils to the inner and outer bands to form a nozzle) are coated separately with the aluminide diffusion coating before the ceramic layer is applied by PVD. See, for example, U.S. Pat. No. 6,442,940 (Young et al), issued Sep. 3, 2002 and U.S. Pat. No. 6,502,400 (Freidauer et al), issued Jan. 7, 2003 for combustor dome assemblies formed from a plurality of parts that are brazed together. These coated parts are then typically machined to remove the coating where the parts are to be joined to and then brazed to the supporting structure to provide the complete component protected by the thermal barrier coating.
Though significant advances have been made in improving the durability of thermal barrier coatings applied by PVD techniques, such coatings will typically require repair under certain circumstances, particularly gas turbine engine components that are subjected to intense heat and thermal cycling. The thermal barrier coating of the turbine engine component can also be susceptible to various types of damage, including objects ingested by the engine, erosion, oxidation, and attack from environmental contaminants, that will require repair of the coating. The problem of repairing such thermal barrier coatings is exacerbated when the component comprises an assembly of individually PVD coated parts that are machined and then brazed to a supporting structure or the like, as, for example, in the case of a combustor dome assembly. In removing the PVD-applied thermal barrier coating (e.g., by grit blasting), some or all of the underlying aluminide diffusion coating can be removed as well. Repairing or reapplying this aluminide diffusion coating while the component is in an assembled state is usually difficult, expensive and impractical.
Even more significant is the difficulty in repairing or reapplying the ceramic layer by PVD techniques while the component is an assembled state. Because of the processing conditions (usually heat) under which PVD techniques are carried out, repairing or reapplying the ceramic layer by PVD (especially EB-PVD) techniques can damage the brazed joints of the assembled component, as well as the supporting structure to which the parts are joined by brazing. As a result, the component is usually disassembled into its individual parts and then the PVD-applied thermal barrier coating is stripped or otherwise removed from the aluminide diffusion coating, such as by grit blasting. The thermal barrier coating can then be reapplied by PVD techniques to the individual stripped parts (with or without prior repair of the underlying aluminide diffusion coating), followed by machining and rebrazing of these PVD recoated parts to the supporting structure to once again provide a complete component. Such a repair process can be labor-intensive, time consuming, expensive and impractical.
In some instances, it can also be desirable to apply a thermal barrier coating by plasma spray (particularly air plasma spray) techniques to the metal substrate of the turbine engine component where the underlying metal substrate has an aluminide diffusion coating. Plasma spray techniques for applying the thermal barrier coating would also be desirable in repairing damaged PVD-applied thermal barrier coatings because the conditions under which plasma spray coatings are applied does not damage brazed joints and would allow the damaged thermal barrier coating to be repaired without disassembly of the component. However, for plasma spray-applied thermal barrier coatings to properly adhere, typically an overlay alloy bond coat layer (e.g., MCrAlY) needs to be applied to the aluminide diffusion coating. However, applying this overlay alloy bond coat layer to an aluminide diffusion coating by plasma spray techniques, especially air plasma spray techniques, is not without problems. In many instances, plasma spray-applied overlay alloy bond coats will not consistently adhere to the surface of the aluminide diffusion coat layer. This also makes it difficult to use plasma spray techniques in place of PVD techniques to repair a damaged PVD-applied thermal barrier coating.
Accordingly, it would be desirable to provide a method for repairing such components having PVD-applied thermal barrier coatings that reduces the cost and time of such repairs and can be employed on a wide variety of turbine engine components, such as combustor deflector plate assemblies and combustor nozzles. It would be further desirable to provide a method capable of applying a thermal barrier coating by plasma spray techniques to a metal substrate that has an overlaying aluminide diffusion coating.